Shear birefringence measurements on polymer thin films deposited on quartz resonators

We have measured the second-order stress birefringence in a 2 mu m film of poly-(methyl-methacrylate) sandwiched between the top electrode of a quartz resonator and a second? semitransparent gold layer. The gold-film-gold system forms an optical Fabry-Perot resonator with characteristic dips of reflectivity at certain angles of incidence, which are the coupling angles of the different optically resonant modes. When modulating the driving voltage of the quartz resonator with a frequency in the kHz range and referencing the detection electronics to the modulation, shifts in the coupling angle in the range of 10(-3)degrees are easily detected. They result from a pseudo-de component in the stress-optic signal, which is proportional to the square of the shear strain. This second-order stress birefringence is related to normal stresses in the film via the stress-optic law. We compare our findings to predictions from the theory of finite elasticity. (C) 1998 American Institute of Physics. [S0021-8979(98)03703-7].